I am trying to determine what size and pressure to run the chamber of my in-the-works paintball gun at in order to achieve a higher efficiency than a regular paintball marker.

To do this, I need to know how much co2 is used in a regular marker per shot. Co2 runs at about 850 psi at room temp, and a 12 oz bottle provides about 500-600 shots. I would like to achieve a similar amount, but in a home-made gun.

This is complicated because a 12 oz tank does not have 12 oz of Co2 in it, and because the liquid co2 expands.

Hailfire753 wrote:I am trying to determine what size and pressure to run the chamber of my in-the-works paintball gun at in order to achieve a higher efficiency than a regular paintball marker.

I kind of doubt that you'll be able to make a gun that is more efficient than a commercial marker. The marker makers have several advantages;
1. They know what they are doing.
2. They can have any part they want manufactured.

They also eject a lot of high pressure Co2 from the bolt ( I am trying to best the tipmann 98 ) The design I am using does waste some pilot air, but it is used to power to mag, like the a5.

Commercial markers are limited to low-flow designs, with high pressure co2 to counter. Higher pressure = more pilot air wasted. I am using a design with 3/4" of flow all the way through. If I use low pressure, less total air will be wasted.

Potatoflinger, I would appreciate if you game me the link to that calculator.

Still, I would need to know the actual amount of liquid co2 contained in a 12 oz bottle, wouldn't I?

Having a good valve and flow and thus needing lower pressures for a certain fps is an good idea BUT:
Then instead of dropping the pressure, decrease chamber size. (any gun that has half the chamber size, but double the pressure will perform better, so you will need even less chamber)
But, your main reason of dropping pressure was to dump less pilot air, solution: do not have any pilot at all.
Also for real efficiency, do not dump the whole chamber after the projectile leaved the barrel.
In the end you will come out to something similar as the HEAR valve that clide's GB semi used. No pilot air, not dumping whole chamber.

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Till the day I'm dieing, I'll keep them spuddies flying, 'cause I can!

don't know if you found the info you are looking for, this is from something i read a long time ago and maybe it will help you.

A "full" tank contains about 34% liquid CO2. If it is filled any more, the CO2 will become very sensitive to temperature changes, with a small increase in temperature causing a large increase in pressure. This is a dangerous situation which is avoided by only partially filling the CO2 bottle.

One cubic inch of water weighs 0.577 oz and the specific gravity of liquid CO2 is 1.977 gm/cc so one ounce of liquid CO2 has a volume of 0.877 cubic inches. CO2 bottles generally have a full-fill to volume ratio of about 2.57 cubic inches per ounce of CO2, so that one ounce of CO2 will take up 0.877/2.57 = 34% of the total volume of the bottle.

The figure of 68% is often quoted as the volume of liquid in a full bottle, but this error probably stems from translating "ounce" into volume using water as the standard. Water is 1.00 gm/cc, or about half the density of liquid CO2 so that if a CO2 bottle is filled to its rated capacity with water, it will be 68% liquid by volume.